This invention generally relates to a method for preparing ceramics and more particularly to a combinatorial method for preparing electron emissive ceramic materials for lamp cathodes.
The standard emissive coating currently used on a majority of cathodes of commercial fluorescent lamps contains a mixture of barium, calcium, and strontium oxides (xe2x80x9cthe triple oxide emissive mixturexe2x80x9d). Because these oxides are highly sensitive to CO2 and water, they are placed on the lamp cathodes initially as a mixture of barium, calcium and strontium carbonates in a slurry suspension containing a binder and a solvent. The mixture of carbonates is then xe2x80x9cactivatedxe2x80x9d during the manufacturing process by resistively heating the cathodes until the carbonates decompose, releasing CO2 and some CO, and leaving behind the triple oxide emissive mixture on the lamp electrode. It is believed that barium in barium oxide, in some form, is primarily responsible for the electron emission. It is also known to add a small amount of Al, Hf, Zr, Ta, W and Th dopants to the triple oxide emissive mixture, as discussed in U.S. Pat. No. 3,563,797 to Young, incorporated herein by reference in its entirety.
The triple oxide emissive mixture suffers from several disadvantages. Lamps having cathodes coated with the triple oxide emissive mixture have a higher than desired work function, which leads to a higher than desirable cathode fall voltage.
Other emissive materials for fluorescent lamps are also known. For example, U.S. Pat. No. 4,319,158 to Watanabe, incorporated herein by reference in its entirety, discloses an emissive material which comprises yttrium oxide and lanthanum oxide. This emissive material may be used in combination with the triple oxide or barium tungstate emissive materials. Furthermore, it has been previously suggested in U.S. Pat. No. 4,031,426 to Kern, incorporated herein by reference in its entirety, to substitute the triple oxide emissive mixture with barium tantalate emissive materials having various barium to tantalum ratios. While the materials of Watanabe and Kern have longer lifetimes than the triple oxide material, they have a lower efficacy than the triple oxide.
The work function of the emissive material depends on a variety of factors, such as activation schedule, morphology, composition and stoichiometry, among others. Therefore, a variety of various emissive materials have been prepared and screened in order to obtain an emissive material with a low work function. The prior art methods prepare and measure the emissive material of a given composition and stoichiometry one sample at a time. Therefore, these prior art methods are slow and inefficient. The present invention is directed to overcoming or at least reducing the effects of one or more problems set forth above.
In accordance with one preferred aspect of the present invention, there is provided a method of making an electron emissive material, comprising providing a plurality of pixels of the electron emissive material, each pixel having at least one different characteristic from any other one of the plurality of pixels, and measuring at least one property of each pixel.
In accordance with another preferred aspect of the present invention, there is provided a method of determining a work function of a plurality of pixels of an electron emissive material, comprising providing an array of pixels of a first material on a first substrate, each pixel having at least one different characteristic from any other one of the plurality of pixels, and measuring the work function of each pixel on the first substrate using a work function measurement device.
In accordance with another preferred aspect of the present invention, there is provided a Kelvin probe combinatorial testing system, comprising a Kelvin probe apparatus, a first substrate adapted to support a plurality of pixels of a material to be tested, each pixel having at least one different characteristic from any other one of the plurality of pixels, and a computer electrically connected to the Kelvin probe apparatus containing software which analyzes a work function measured on the plurality of pixels and which provides a visual, electronic or printed output of the work function of each pixel.